Conventionally, as a hydraulic piston pump, an axial piston pump has been widely employed as a fixed capacity type pump or a variable capacity type pump.
In general, in a hydraulic piston pump, oil is, in an absorption process, absorbed from an absorption port of a valve plate into a cylinder bore through a cylinder port of a cylinder bore formed in a cylinder block. Further, in a discharge process, pressure oil in the cylinder bore is discharged into a discharge port of the valve plate through a cylinder port. The discharged pressure oil is supplied to a hydraulic pressure system having a specific system pressure, an actuator, or the like.
In an area in which the cylinder port is switched from the absorption port to the discharge port, a chamber pressure of the cylinder bore is an absorption pressure until the time when the cylinder port is at a position corresponding to a bottom dead center of a piston inside the cylinder bore. In a pre-compression section between the absorption port and the discharge port, the piston slides from the bottom dead center toward a top dead center, and the chamber pressure of the cylinder bore is increased so that the pressure is increased to a pressure close to the system pressure. Thereafter, the cylinder port is coupled with the discharge port, so that the pressure oil inside the cylinder bore is discharged into the discharge port with compression by the piston.
In the pre-compression section, a pressure increment amount by which the chamber pressure of the cylinder bore is increased is constant. Thus, when the system pressure in the hydraulic pressure system or the like to which the pressure oil is supplied from the discharge port changes, oil pressure at the discharge port, that is, the system pressure, changes. When the cylinder port is coupled with the discharge port in this state, a pressure difference between the chamber pressure of the cylinder bore which corresponds to the system pressure before the change and the system pressure after the change becomes large, so that pressure change inside the cylinder bore becomes drastic. This becomes a cause of vibration and noise in the hydraulic piston pump. The vibration and noise generated in the hydraulic piston pump adversely affect operational environment.
As a method to prevent this, the pre-compression section is decreased in some cases. In such cases, however, backflow of the system pressure into the cylinder bore occurs, and erosion may be generated in the cylinder bore, and/or cavitation may be generated to cause vibration and noise.
As pumps in which vibration and noise are prevented without decreasing the pre-compression section, there have been proposed a hydraulic pump in which first and second conduits are formed on a pre-expansion section in which the discharge port is switched to the absorption port and the pre-compression section, respectively, so that the respective conduits communicate with each other through a check valve (see Patent document 1) and a low noise hydraulic pump in which a check valve timing device is provided on the pre-compression section (see Patent document 2).
The hydraulic pump disclosed in the Patent document 1 is configured, as shown in FIG. 14, such that a first conduit 44 is formed on a pre-expansion section θ1 on a valve plate 40, and a second conduit 45 is formed on a pre-compression section θ2. An opening position of the first conduit 44 is formed on a portion at which a cylinder port 43 of a cylinder bore formed in a cylinder block communicates with the first conduit 44 and is formed at a position immediately before the cylinder port 43 communicates with an absorption port 41.
An opening position of the second conduit 45 is formed on a portion at which a cylinder port 43 communicates with the second conduit 45 and is formed at a position immediately after the cylinder port 43 is disconnected from the absorption port 41. The first conduit 44 and the second conduit 45 are coupled with an accumulator 50 through check valves 46, 47, respectively. The check valve 46 allows flow from the first conduit 44 side to the accumulator 50, and the check valve 47 allows flow from the accumulator 50 to the second conduit 45 side.
When the cylinder port 43 finishes the communication with a discharge port 42 and enters the pre-expansion section θ1, the chamber pressure inside the cylinder bore is decreased. When the cylinder port 43 communicates with the first conduit 44, pressure oil inside the cylinder bore whose pressure is decreased in the pre-expansion section θ1 enters the accumulator 50 through an oil path 48 and the check valve 46. The chamber pressure inside the cylinder bore is further decreased, while the pressure inside the accumulator 50 is increased to the chamber pressure inside the cylinder bore. Thus, the pressure difference between the chamber pressure inside the cylinder bore and the absorption pressure of the absorption port 41 can be decreased.
When the cylinder port 43 ends the communication with the absorption port 41 and the piston reaches the bottom dead center, the cylinder port 43 communicates with the second conduit 45. At this time since the chamber pressure inside the cylinder bore is the absorption pressure, the pressure oil inside the accumulator 50 enters the cylinder bore through an oil path 49, the check valve 47, and the second conduit 45 to increase the chamber pressure inside the cylinder bore.
Consequently, the pressure difference between the chamber pressure inside the cylinder bore and the system pressure of the discharge port 42 is decreased. When the cylinder port 43 communicates with a throttle path 42a, the flow rate from the discharge port 42 into the cylinder bore is decreased, so that pulsating due to the discharge flow rate can be decreased.
The low noise hydraulic pump disclosed in the Patent document 2 is formed so as to have such a constitution shown in FIG. 15. FIG. 15 shows a perspective view partly broken away of a valve plate 60 in which a communication hole 64 is formed in a pre-compression section provided between an absorption port 61 and a discharge port 62 and in which a check valve 66 is incorporated in the communication hole 64. A check valve chamber 65 is formed in a lower end side of the communication hole 64. The inner diameter of the check valve chamber 65 is formed so as to be slightly larger than the outer diameter of the check valve 66 such that the check valve 66 is reciprocatable inside the check valve chamber 65.
The check valve chamber 65 has an opening on a check valve pocket 67 formed on a matching face of a valve block 68 of the hydraulic pump. The check valve pocket 67 is formed so as to be smaller than the check valve chamber 65 such that the check valve 66 stays along the surface of the valve block 68. A pressure oil path 69 communicating with the check valve pocket 67 is formed inside the valve block 68 and communicates with the discharge port 62.
The check valve 66 is formed of a thin disk having a plurality of holes 70 positioned about a center hole 71 of the disk. These holes 70, 71 are respectively formed such that a desired amount of flow is made to pass through a check valve assembly 63.
When the cylinder port of a cylinder bore is spaced apart from the absorption port 61, the cylinder port immediately communicates with the communication hole 64. The chamber pressure in the cylinder port at this position is lower than the system pressure in the discharge port 62. Thus, the pressure oil in the discharge port 62 enters through the path 69 to allow the check valve 66 to press the valve plate 60.
At this time, the holes 70 formed having the same center are closed, and the pressure oil entering through the path 69 is introduced into the cylinder bore through the central hole 71. Thus, by the pressure oil introduced through the communication hole 64, the chamber pressure in the cylinder bore is increased.
When a piston is pressed by means of a cam plate or the like to be lowered during rotation of the cylinder block, the chamber pressure in the cylinder bore increases. When the chamber pressure exceeds the system pressure in the discharge port 62, the pressure oil in the cylinder bore presses the check valve 66 downwardly. At this time, the pressure oil entering the check valve chamber 65 from the cylinder bore through the communication hole 64 can pass all of the holes 70, 71 and enter the check valve pocket 67.
Thus, a large amount of pressure oil can enter the discharge port 62, and when the cylinder port and the discharge port 62 communicate with each other, the chamber pressure in the cylinder bore can be equal to the system pressure in a steady flow rate state.
Patent document 1: Japanese Patent Application Laid-open No. 9-317627
Patent document 2: WO 97/22805